The bile duct is a duct guiding bile produced in the liver into the duodenum and is roughly classified into the intrahepatic bile duct in the liver and the extrahepatic bile duct outside the liver. The extrahepatic bile duct is linked to the gallbladder for temporarily storing and concentrating bile via the gallbladder duct. The extrahepatic bile duct at which the extrahepatic bile duct and the gallbladder duct merge is called the common bile duct, and the intrahepatic bile duct, the extrahepatic bile duct, and the gallbladder are collectively referred to as the biliary tract.
Most bile duct carcinomas are cancerous biliary epithelial cells covering the lumen; chemotherapy and radiotherapy have a little effect on the carcinomas and surgical resection with early detection is only curative treatment. However, there is no symptom for early bile duct carcinoma, for example, extrahepatic bile duct carcinoma is often found in a state of advanced cancer since symptoms, such as jaundice and itching, do not occur until the bile duct is obstructed by the progress of the carcinoma with bile flowed back into the blood vessel. On the other hand, intrahepatic bile duct carcinoma does not quite obstruct the extrahepatic bile duct, and thus, carcinoma often progresses while remaining asymptomatic without jaundice symptoms. According to statistics on cancer death rates by site in Japan in 2014 disclosed by the Center for Cancer Control and Information Services, National Cancer Center, the number of people dying of gallbladder/bile duct carcinoma amounted to 18,117, and the 5-year relative survival rate by site in 2003 to 2005 was 22.5% for males and 19.9% for females, being the second worst after pancreatic cancer. The bile duct is closely related to important organs, such as the liver and the pancreas, and thus, the metastasis of carcinoma to these organs contributes to the aggravation of prognosis.
Less-invasive abdominal ultrasonography and hematological examination are generally used for the diagnosis of biliary tract carcinoma (Non-patent Document 1). The rate of visualization of bile duct carcinoma by abdominal ultrasonography ranges from 21 to 90%, and it is considered problematic that the visualization rate reduces when the occupation site is the lower bile duct. Hematological examination using an increase in a tumor marker, such as CEA or CA19-9, as an index is carried out, but these tumor markers do not enable the early detection of biliary tract carcinoma and have a problem with diagnostic accuracy. Recently, a method has been reported for detecting bile duct carcinoma using the expression level of specific microRNA (miRNA) as an index (Patent Document 1).
Carcinoembryonic antigen (CEA) is known to be one of the embryonal antigens produced from carcinoma cells and be a glycoprotein having a molecular weight of around 200,000. CEA has 10 or more structurally similar subfamilies, and CEACAM1 is known as one of them. CEACAM1 in serum is reported to be usable as a marker for pancreatic carcinoma diagnosis (Non-patent Document 2), but the relation between CEACAM1 and extrahepatic bile duct carcinoma or the like has not previously been known.